Fiber optic telemetry schemes are an economic approach to sensing seismic sensor arrays, which typically extend over long distances, sometimes several miles. In some seismic sensor systems, the optical sensor signals are outputs of interferometers, and are called fiber optic interferometric sensor systems. In such systems, demodulation electronics are used to process the signals. Processing the signals, however, is complicated by strain on the interferometer, which causes light propagating through the arms of the interferometer to cycle through fringes, the rate of such cycling commonly referred to as the xe2x80x9cfringe rate.xe2x80x9d
Sensors in close proximity to the excitation energy source, like an acoustic source such as an air gun, generate high fringe rates compared to sensors that are far from the excitation energy source. The communication channels carrying signals from sensors located in the proximity of the excitation energy source are theoretically capable of carrying the multiplexed signals. The signal processors, for example demodulators, become overloaded and tend to drop some of the signals; under such circumstances signal information is lost. Therefore, the communication channels are designed for the close proximity loads, and the communication channels carrying signals from sensors located far from the excitation energy source are under-utilized. On the other hand, when the seismic sensors are located so far away from the excitation energy source the resulting signals are so weak that they become excessively attenuated over the communication channels that they cannot be recorded on the recording system and are thus lost. Therefore, there is a need for a more economic approach to processing high and low fringe rate signals without losing information and without requiring a large number of demodulators.
In view of the described problems in processing the seismic sensor interferometric signals, it is desirable to prevent signals from dropping out caused by overloading of some of the demodulators. In addition, it is desirable to optimally utilize the demodulators by routing signals to a high performance demodulator capable of processing signals having high fringe rates. Either of these needs, individually or both simultaneously, is achieved in various embodiments of the claimed invention by routing signals to a high performance demodulator only when necessary, i.e., on a need basis only, then fewer high performance demodulators are required to achieve optimum performance, and thereby cost less than traditional systems.
According to one aspect of the present invention, an apparatus is provided for optimizing bandwidth utilization of demodulators in a system for seismic signal processing, wherein the apparatus comprises a first demodulator having a first bandwidth and first optical demodulator inputs. The apparatus further comprises a second demodulator having second optical demodulator inputs and a second bandwidth, wherein the second bandwidth is greater than the first bandwidth. Still further, the apparatus comprises an optical switch having optical inputs coupled to optical fibers from seismic telemetry, optical outputs coupled to the first optical demodulator inputs and the second optical demodulator inputs, and a switch control input electronically coupled to a fringe rate detector output. The apparatus also comprises a fringe rate detector having a fringe rate detector input coupled to the optical fibers from seismic telemetry and a fringe rate detector output coupled to the switch control input. In addition, the optical fibers bear seismic optical signals having fringe rates; and, overall, the optical signals are switched among the demodulators in dependence upon the fringe rates.
According to another aspect of the invention, a method is provided of optimizing bandwidth utilization of demodulators in a system for seismic signal processing, wherein the method comprises routing optical signals to a first demodulator, sensing a fringe rate for each of the optical signals, and detecting a signal line having fringe rates exceeding a threshold fringe rate input for the first demodulator. The method further comprises rerouting the signal line to a second demodulator, detecting a condition of the signal line, and rerouting the signal line from the second demodulator back to the first demodulator when the signal line has fringe rates below the threshold fringe rate input for the first demodulator.
According to still another aspect of the invention, a system is provided of optimizing bandwidth utilization of demodulators in a system for seismic signal processing, wherein the system comprises a means for routing optical signals to a first demodulator, a means for sensing a fringe rate for each of the optical signals, and a means for detecting a signal line having fringe rates exceeding a threshold fringe rate input for the first demodulator. The system further comprises a means for rerouting the signal line to a second demodulator, a means for detecting a condition of the signal line, and a means for rerouting the signal line from the second demodulator back to the first demodulator when the signal line has fringe rates below the threshold fringe rate input for the first demodulator.
In still further aspects of the invention, an apparatus, method, and system are provided for routing the optical signals to a demodulator resulting in demodulated digital electronic signals, wherein each of the demodulated signals includes a dynamic range digitally encoded in a fixed number of bits. In addition, the apparatus, method and system further involves selecting a portion of the fixed number of bits for each of the demodulated signals resulting in adjusted signals, and outputting the adjusted signals to a recorder.